Analog signal to frequency converters are well known. Analog signals generated by various devices such as sensors are often converted into corresponding digital signals because of the convenience and accuracy of digital signal processing. Often the sensor is remote from the computer, therefore, the signal has to be transmitted to the computer. Accurate signal transmission with analog signals is very difficult. In order to improve the accuracy, the signal is converted into a frequency by means of an analog signal-to-frequency converter, In the digital form, the signal can be transmitted to the computer substantially without interference. In the computer, a digital word is formed out of the frequency, for example, by counting the pulses during a predetermined time. This type of transmission offers the advantage that the frequency is not disturbed by the attenuation of a transmission cable or similar influences.
A well known type of converter is the charge balanced voltage-to-frequency type. This type of converter generally includes a capacitor, connected with an operational amplifier to form a current integrator that is cyclically charged, first in one direction, second in the opposite direction (i.e., charged and discharged). This is done at a frequency which changes linearly with the input voltage applied from the sensor. The net charge applied to the capacitor during each cycle is zero, a result achieved by charging the capacitor in the first direction for a predetermined period of time. In response to the end of that time, the capacitor is charged in the opposite direction until a predetermined level is reached. The rate at which the capacitor is charged in the first direction is controlled by the sum of a current derived from the input voltage and a fixed current source. The rate at which the capacitor is discharged is determined by a current derived from the input voltage. Therefore, the frequency of the charge and discharge cycles is a direct function of the input voltage magnitude.
In another charge rebalancing scheme, an input voltage from the sensor builds up a charge on the capacitor for the integrator. The comparator output initiates a feedback circuit which transmits pulse of current of the opposite polarity of the input signal to the capacitor. The number of pulses required to balance the capacitor is indicative of the magnitude of the sensor signal and a bit is added to the digital word which is output in order to indicate polarity of the incoming signal.
For many applications it is required that the converter be bidirectional. This requires that the balancing current source be bidirectional also. Many bidirectional converters have not been capable of providing the performance goals of linearity and bias stability required in critical applications. One reason for the lack of accuracy in such converters is that positive and negative rebalance current pulses are unequal or that consecutive rebalance pulses do not contain like amounts of energy. In these types of converters, rebalance current pulses must be very accurate, of opposite polarity, and they must match during calibration where any mismatch appears as an offset which is inseparable from the real offset.
Therefore, it is an object of this invention to provide a current source which outputs currents equal in magnitude but opposite in polarity.
It is also an object of this invention to provide a current source which can be used with most charge rebalancing digitizers.